1. Field of the Invention
The present invention relates to an ignition timing control system for controlling ignition timing of an internal combustion engine, a method thereof, and an engine control unit.
2. Description of the Related Art
A conventional ignition timing control system for an internal combustion engine is disclosed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2005-315161. The engine is provided with a variable valve actuating mechanism that opens and closes intake valves and changes the maximum lift of the intake valves. In this engine, by controlling the maximum lift of the intake valves using the variable valve actuating mechanism, the amount of intake air is controlled. Further, the conventional control system determines ignition timing in the following manner: the engine speed and the amount of intake air are detected, and based on the detected engine speed, the maximum value of the amount of intake air that can be drawn when the engine is at the detected engine speed (hereinafter referred to as “the maximum intake air amount”) is calculated. Further, the ratio between the detected amount of intake air and the calculated maximum intake air amount is calculated, and the ignition timing is determined according to this ratio and the detected engine speed.
On the other hand, one of known variable valve actuating mechanisms is configured, as shown in FIG. 7, such that the opening timing of the intake valves is advanced and the closing timing of the same is retarded as the maximum lift Liftin is larger. FIG. 35 shows an example of control of the ignition timing in the case where this type of variable valve actuating mechanism is employed. As shown in FIG. 35, as the maximum lift Liftin is larger, the intake air amount is increased to accordingly increase the output torque, which tends to make knocking more liable to occur, and hence, to avoid this problem, the ignition timing is determined to be retarded.
Further, when the maximum valve lift is a first predetermined value La, the ignition timing is determined to be a first predetermined ignition timing Ia, whereas when the maximum valve lift is larger than the first predetermined value La, it is determined to be advanced as the maximum lift Liftin is larger, and when the same is larger than a second predetermined value Lb larger than the first predetermined value La, it is determined to be a second predetermined ignition timing Ib more advanced than the first predetermined ignition timing Ia. The reason for this is as follows: As the maximum lift Liftin is larger, the opening timing of the intake valves becomes more advanced, whereby the amount of exhaust gases remaining within each cylinder (hereinafter referred to as “internal EGR gas”) increases, and the closing timing of the same becomes more retarded, whereby the amount of air returned into the intake pipe from within the cylinder increases. Thus, as the amount of internal EGR gas increases, the combustion temperature lowers to lower the temperature of the cylinder wall, which lowers the temperature of the mixture, and the compression ratio of the engine lowers due to the increase of the amount of return air. The lowered temperature of the mixture and the reduced compression ratio of the engine makes knocking difficult to occur.
Further, as described above, when the maximum lift Liftin ≧ the first predetermined value La holds, the amount of return air is larger as the maximum lift Liftin is larger. However, by determining the ignition time such that it is more advanced, the combustion ratio of the engine becomes higher, which increases the output torque of the engine. Further, due to the increase in the amount of return air, the actual intake air amount assumed when the maximum lift Liftin is equal to the second predetermined value Lb becomes equal to a predetermined value GREF, which is equal to the intake air amount to be assumed when the maximum valve Liftin is equal to the first predetermined value La. As a consequence, as shown in FIG. 36, when the amount of intake air is equal to the predetermined value GREF, there are two ignition timings suited thereto: first ignition timing Ia and second ignition timing Ib.
The conventional ignition timing control system described above, however, determines ignition timing according to the intake air amount. Therefore, assuming that when the intake air amount is equal to the predetermined value GREF, the first ignition timing Ia on the retarded side is determined as the ignition timing so as to avoid occurrence of knocking, if the actual valve lift Liftin is equal to the second predetermined value Lb, knocking would not occur even if the ignition timing is determined to be the second ignition timing Ib. In spite of this, since the ignition timing is determined to be the first ignition timing Ia, it is impossible to obtain a high combustion efficiency, which reduces output torque of the engine.
To avoid such an inconvenience, it can be envisaged to control the ignition timing according to the maximum lift Liftin and the engine speed. In this case, however, there arises the following problems: When determining the ignition map by searching a map according to the maximum lift Liftin, in a region of the maximum lift Liftin (hereinafter referred to as “the knock lift region”) where load on the engine is high and knocking is liable to occur, to prevent occurrence of knocking, it is necessary to increase the number of grid points to which map values of ignition timing are set. On the other hand, even if the maximum lift Liftin is the same, the intake air amount varies with the engine speed, and hence as shown in FIG. 37, as the engine speed varies between low and high, there are different knock lift regions (areas enclosed by broken lines) which as a whole forms a very wide region. As a result, it is necessary to set map values for the very wide knock lift region as described above, the number of settings of map values (number of grid points indicated by black dots) becomes too large, and hence the capacity of a memory required by the ignition timing control system becomes too large, resulting in an increase in the manufacturing costs of the ignition timing control system.